POLLUTION PREVENTION PROGRAM

NORTH CAROLINA DEPARTMENT OF ENVIRONMENT, HEALTH, AND
NATURAL RESOURCES

SILVER RECOVERY SYSTEMS AND WASTE REDUCTION IN
PHOTOPROCESSING

There are several reasons to be interested in the recovery of silver
from photoprocessing waste. Silver is a valuable natural resource of
finite supply, it has monetary value as a recovered commodity, and it's
release into the environment is strictly regulated. In photoprocessing,
silver compounds are the basic light-sensitive material used in most of
today's photographic films and papers. During processing, particularly in
the fixing bath or bleach-fix, silver is removed from the film or paper
and is carried out in the solution, usually in the form of a silver
thiosulfate complex.

Major sources of recoverable silver are: photoprocessing solutions,
spent rinse water, scrap film, and scrap printing paper. As much as 80
percent of the total silver processed for black and white positives and
almost 100 percent of the silver processed in color work will end up in
the fixer solution. Silver is also present in the rinse water following
the fixer or bleach-fix due to carry-over.

Economic considerations include initial equipment cost, the amount and
value of silver recovered, and the return on investment. Space and energy
requirements, day-to-day attention required, maintenance, and reliability
are also important. It is necessary to know the amount of silver available
for recovery, the total volume of fixer and bleach-fix solutions used in
processing, and the expected performance of the recovery method under
consideration.

Several technologies exist for recovering silver onsite. The most
common methods of onsite recovery from the fixer and bleach-fix processing
solutions involve metallic replacement, electrolytic recovery and chemical
precipitation. Ion exchange and reverse osmosis are other methods that can
be used alone or in combination with conventional silver recovery systems.
However, these are generally considered suitable only for dilute solutions
of silver. A silver recovery system can be devoted to a single process
line or can be used to remove silver from the combined fixer from several
process lines in a plant.

The most widely used silver recovery method for large operations is
electrolysis, where the silver is recovered from solution by
electroplating it on a cathode, Fig. 1. A controlled, direct electrical
current is passed between two electrodes suspended in the silver-bearing
solution. Silver is deposited on the cathode in the form of nearly pure
silver plate. The cathodes are removed periodically, and the silver is
stripped off for sale or reuse. While this method requires a substantially
larger capital expenditure and needs an electrical connection it does have
the advantage over other methods in that it yields virtually pure silver.
This results in lower refining and shipping costs and it does not
contaminate the fixer, thereby permitting its reuse for some processes.
When properly operated, 95 percent of the potential available silver can
be recovered. Combining electrolytic silver recovery with in-situ ion
exchange can result in more than 99.5 percent silver recovery efficiency.

A recirculating electrolytic recovery system has advantages over
systems that only remove silver. Silver is removed from fixer solution by
the recovery cell which is connected "in line" as part of a
recirculation system. Fixer solution reclaimed by electrolytic silver
recovery can have limited reuse in the photoprocess. By recirculating the
desilvered fixer to the in-use process tank, less fresh fixer solution is
needed to replenish the bath. Fixer replenishment can be reduced 20
percent or more without degradation of product quality. Chemical
replenishment can be managed through the frequent and consistent use of
test strips. A properly designed recirculating system can lower the silver
in the fixer from a concentration of 1 ounce/gal. to 1 ounce/100 gals. The
amount of silver carried over to the rinse water is similarly reduced.

Metallic replacement requires little capital expenditure for equipment
and requires only a few simple plumbing connections. The equipment
consists of a plastic container, a plastic-lined steel or stainless steel
drum filled with metal, usually steel wool, and some plastic hose and
plumbing connections.

See Fig. 2. Silver is recovered when the silver-bearing solution flows
through the cartridge and makes contact with the steel wool. The iron goes
into solution as an ion, and the metallic silver is released as a solid to
collect in a sludge at the bottom of the cartridge or is deposited on the
steel wool. The yield a user can expect is determined by the silver
concentrations in solution, the volume of solution that is run through the
cartridge, and the care with which the operation is managed. When silver
is no longer effectively removed, the silver-bearing sludge is sent to a
refiner who will refine it and pay the customer for the recovered silver.

The disadvantages of these two methods are that neither can recover
more than 95 percent of the silver from concentrated solutions,
effectively treat dilute wastewater, nor remove other metals from the
effluent.

Another option is chemical precipitation with sodium sulfide, sodium
borohydride or sodium dithionite. This can remove virtually 100 percent of
the silver and most other metals from photographic effluent. With the
addition of alkaline sodium sulfide and the resulting precipitation of
silver sulfide, levels of soluble silver below 0.1 mg/L are possible.
However, the more difficult part of the process is the separation of the
precipitate from the liquid. Total silver levels of 0.5 to 1.0 mg/L are
usually obtained due to filtration limitations. This process requires only
a small capital expenditure and uses chemicals which are relatively
inexpensive. It is not as widely used as the electrolytic or metallic
replacement methods because of the inconvenience of handling large amounts
of chemicals, the separation process required, and the problem of
concentrating finely precipitated silver sulfide particles into a sludge
that can be dried and refined. Also, careful pH control is required to
avoid generation of highly toxic hydrogen sulfide gas.

Figure 3. Ion exchange system.

Ion exchange is generally used for effective recovery of silver from rinse
water or other dilute solutions of silver. The ion exchange method
involves the exchange of ions in the solution with ions of a similar
charge on the resin. The soluble silver thiosulfate complex is exchanged
with the anion on the resin. This exhaustion step and is accomplished by
running the solution through a column containing the resin, Fig. 3. For
large operations, the next step is the regeneration step in which the
silver is removed from the resin column with a silver complexing agent
such as anmnonium thiosulfate. This step includes several backwashes to
remove particulate matter and excess regenerant before the next exhaustion
step is initiated. Silver is then recovered from the thiosulfate
regenerant with an electrolytic recovery cell. For smaller operations an
alternative to performing the regeneration step onsite would be to remove
the resin from the column and send it to a refiner for silver reclamation.
Important factors in considering an ion exchange system for silver
recovery are; selection of the resin, flow rate of the silver-bearing
solution, column configuration, and selection of the regenerant. It has
been demonstrated that the use of ion exchange can reduce the silver
concentration in photographic effluent to levels in the range of 0.5 to 2
mg/L and can recover over 98 percent of the available silver. If this
method is used as a tailing method after primary recovery by electrolysis,
levels in the range of 0.1 to 1 mg/L can be obtained.

Reverse osmosis (RO) is also used for dilute solutions. RO uses high
pressure to force the silver-bearing solution through a semipermeable
membrane to separate larger molecules, such as salts and organics, from
smaller molecules like water. The extent of separation is determined by
membrane surface chemistry and pore size, fluid pressure, and wastewater
characteristics. For removal of silver, after-fix rinse water is
flow-equalized, filtered, and pumped through an RO unit. Once the silver
is separated from the water in this manner it can be recovered by
conventional means such as metallic replacement, electrolytic recovery or
chemical precipitation. Operating problems include fouling of the membrane
and biological growth.

Evaporation is another option for managing waste photographic
solutions. The wastewaters are collected and heated to evaporate all
liquids. The resulting sludge is collected in filter bags. These bags can
be sent to a silver reclaimer for recovery. The major advantage of the
evaporation technique is it achieves "zero" water discharge.
This method would be useful to operations that do not have access to sewer
connections or wastewater discharge. A disadvantage is that the organics
and ammonia in the waste solution may also be evaporated, creating an air
pollution problem. A charcoal air filter may be necessary to capture the
organics. Filter purchase, disposal, and electrical power add to operating
costs.

An alternative to onsite recovery is to collect the bleach/fix in
containers and have a silver recovery contractor haul it away to reclaim
the silver. For this service the photolab may be paid only about 20
percent of the silver value. This low percentage may be partially offset
by its high silver recovery yield. Off site recovery can be done on a
larger, more efficient scale than onsite recovery. The small quantity
generator or the generator who desires a minimum commitment may find
advantages in off site services.

Silver can be recovered from scrap film and paper by soaking the
material in spent fixer solution. Once dissolved in the fixer, the silver
can be recovered through any of the silver recovery processes used by the
lab. There are also businesses which will buy scrap photographic film and
paper from the photoprocessor.

There are additional actions that should be considered by photolabs to
minimize waste.

Inventory of chemicals should be controlled so that they are used
before their expiration date.

Solutions should be made up only in quantities to meet realistic
processing volumes .

Floating lids should be used on developer solution tanks to prevent
evaporation and loss of potency.

Spent rinse water can be treated to restore purity and recycled for
rinsing.

The use of squeegees can reduce considerably the amount of liquid
carried out of the solution by the film.

Common sense safeguards such as keeping the mixing area clean,
avoiding mixing of dry chemicals where airborne particles can cause
contamination of other solutions, and use of separate mixing tanks for
developers will minimize contamination or errors in mixing.

Counter-current rinsing can be used to reduce water consumption. The
basic concept of counter-current rinsing is to use the water from
previous rinsings to contact the film at its most contaminated stage.
Fresh water enters the process only at the final rinse stage.

Reasonable effort has been made to review and verify information in
this document. Neither PNEAC and its partners, nor the technical reviewers and their
agencies, assume responsibility for completeness and accuracy of the information, or its
interpretation. The reader is responsible for making the appropriate decisions with
respect to their operation, specific materials employed, work practices, equipment and
regulatory obligations. It is imperative to verify current applicable regulatory
requirements with state and/or local regulatory agencies.